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Other Terms: Lesser multangular bone, Trapezoid bone, Os trapezoideum
Like the trapezium this is an irregular shaped bone presenting six sides, although smaller. It is positioned in the distal row of carpal bones at the base of the second metacarpal. It is the second smallest carpal bone. It is surrounded on three sides by other carpal bones, the capitate, trapezium, and scaphoid. Its dorsal and ventral surfaces are roughened from the attachment of ligaments. The other four surfaces contain smooth facets for articulation with neighboring bones.
Trapezoid is derived from the Greek word trapeza meaning a four-legged table. The word was extended to mean any four-sided structure with no two sides parallel. Synonymous names used for this bone throughout history are the lesser multangular and pyrimidale.
The trapezoid articulates with four bones: the second metacarpal, scaphoid, trapezium, and capitate bones. Its distal surface forms triangular facet that is convex in the transverse plane with lateral and medial concavities at right angles to this. This facet receives the grooved base of the second metacarpal. On the medial side of the bone is a concave facet for the distal articular surface of the capitate bone. The smooth, convex lateral surface articulates with the trapezium, while the ovoid proximal surface articulates with the scaphoid bone.
The trapezoid ossifies from one center which typically emerges during the fourth to fifth years, usually earlier in females. As with all the carpal bones the time of ossification is variable.